The present invention relates to a process and system for treating wastewater in a constructed wetland facility.
The use of constructed wetlands for the treatment of wastewater is becoming an increasingly popular option especially in rural or lightly developed areas. Constructed wetlands use wetland plants and bacteria to remove pollutants from the wastewater.
Phosphorus removal is a difficult task in any water treatment technology, and wetland technology is no exception. On a per unit area basis, wetlands are not efficient in phosphorus reduction. Treatment wetlands in general are area intensive compared to xe2x80x9cconventionalxe2x80x9d technologies, and the land requirement for wetland phosphorus reduction is typically the largest of all wetland requirements.
Wetlands consist of several components: water, plants, microbiota, plant litter, and soil (or media for subsurface flow wetlands). A large number of transfer and alteration processes occur, but only soil building or sorption provides for net long-term phosphorus storage.
Phosphorus removal by harvesting biomass has not thus far proven feasible. It is difficult to harvest rooted emergent macrophytes in wetlands, and when successful, relatively tiny amounts of phosphorus have been reclaimed in the harvested biomass. One study, for example, reported an average of 2.5 percent of the total phosphorus removal in surface-flow wetlands was achieved by harvest. Floating aquatic plants are somewhat easier to harvest; over 20 percent of the total phosphorus removal was achieved by water hyacinths in another study. Harvesting is labor intensive and costly, which is antithetical to the passive character of wetlands technology. The problem of biomass utilization exacerbates the difficulties.
Because phosphorus is a nutrient, the addition of this element to the wetland stimulates growth and causes increases in the amount of biomass. Phosphorus is used not only to make the plant, it is used to make the plant bigger. This in turn generates more litter. The increase in the biomass cycle is even slower than the initial uptake; one must wait for a cohort to undergo an entire decomposition cycle, which takes many months. The increase in the pool of biomass phosphorus is a short-term process and is presumably reversible.
The two important physical processes for phosphorus removal in wetlands are sedimentation of particulate phosphorus and sorption of soluble phosphorus. Incoming particles may contain phosphorus in available and unavailable forms. If the particulate matter is planktonic, then it may subsequently decompose to release soluble phosphorus. The particles may also contain weakly sorbed phosphorus, which may subsequently desorb. But if the particles contain phosphorus as insoluble minerals or refractory organophosphorus complexes, it maybe permanently removed by the process of sedimentation.
All wetland soils have a capacity to sorb phosphorus, but that capacity is quite variable. This storage may be quickly exhausted in many surface flow treatment wetlands. In contrast, the particulate media in subsurface flow wetlands may be designed to possess a large phosphorus storage via sorption. Iron- and aluminum-rich materials, limestone media, and specially prepared clays have all been employed to enhance this removal mechanism. However, even high storage capacity soils and media will eventually be saturated with sorbed phosphorus. When the saturation point is reached, the wetland will no longer remove phosphorus and must be rebuilt which is expensive and may leave the treatment facility inoperable or inefficient for a period of time.
Therefore, a wetland wastewater treatment system that provides for long term removal of phosphorus and is easily recharged is still needed.
The present invention provides a process and system for enhanced nitrogen removal in a wetland wastewater treatment facility that overcomes the aforementioned problems.
In a preferred embodiment, the invention is a process for improved phosphorus removal from wastewater treated in a constructed wetland wastewater system, the process comprising the steps of: (A) providing a constructed wetland having an inlet end, an outlet end and means for recycling water from the outlet end to the inlet end, the wetlands adapted to facilitate flow of wastewater from the inlet end to the outlet end, the recycling means comprising a media capable of removing phosphorus from water through sorption processes; (B) introducing wastewater to the wetland at or proximate to the inlet end, the wastewater comprising phosphorus containing material; (C) allowing the wastewater to flow from the inlet end to the outlet end; (D) draining water from the outlet end of the wetland; (E) recycling some of the water drained from the outlet end to the inlet end of the wetland such that the recycled water contacts the media; and (F) replacing the media as needed to maintain a phosphorus content in the treated wastewater that is below a predetermined maximum.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.